1. Field of the Invention
The present invention relates generally to temperature monitoring arrangements and, more particularly, is concerned with an arrangement for monitoring the temperatures of water-cooled stator windings of an electric generator.
2. Description of the Prior Art
In electric generators, such as those driven by steam turbines, a tremendous amount of heat is produced during normal operation. Some generators may weigh hundreds of tons and the unchecked production of heat in such machines as caused by mechanical and I.sup.2 R losses would quickly lead to a complete generator failure.
Accordingly, the heat dissipation requirement for a generator is accomplished with a cooling system which utilizes a flow of cooling gas within the generator housing as well as within the rotor and stator structure to remove the produced heat. Some generator designs additionally flow the cooling gas through the stator coils themselves while in another design a cooling liquid such as water is passed through the stator coils for cooling purposes.
In a water cooler system, such as disclosed in U.S. Pat. No. 4,602,872 to Emery et al which is assigned to the assignee of the present invention, water is piped into a circumferential manifold coil ends by means of tubing. The water discharges from the stator coil at the opposite ends where it is collected by tubes feeding into a discharge manifold from which the water is processed and cooled for recirculation. During passage through the stator coil, the water functions to absorb generated heat.
A plurality of temperature sensors, such as thermocouples, are employed for the continuous monitoring of the cooling water to protect the generator against failure. Temperatures are typically monitored with thermocouples connected to the stator coil's coolant discharge manifold for detecting abnormally high temperatures that may result from the occurrence of an abnormal malfunction. A rise in temperature of the cooling water may be indicative of such abnormalities as a broken conductor or a coolant water flow reduction due to a partial blockage of the coolant path.
When stator coil problems develop, such as broken conductors or coolant flow reduction, abnormally high temperatures can be reached in the copper stator winding which carries the generator current. If the problem is not detected early enough, severe damage can result to the current carrying conductors. Because of the high voltages present on the stator coils, the thermocouples cannot be directly connected to the copper coil.
The current prior art method of monitoring the temperature of the coil's coolant is with thermocouples attached to each stator coil discharge water hose manifold nipple or connection. For complete protection of the generator's stator winding, a thermocouple is used to monitor each coil coolant temperature. This is a total of seventy-two points for a thirty-six slot generator. The thermocouples are mounted directly to the coolant discharge manifold, which is at ground potential. The temperature is monitored without penetration of the water system.
Because of the requirements that the thermocouple not penetrate into the water system and that it not be placed at a high voltage location, the thermocoupled is located on the connection nipple where each coil's discharge water flows to the water supply return circuit. A total water blockage to any one coil cannot be detected by a temperature change because of the thermocoupled locations. When a total water blockage occurs, no hot water from the involved coil will flow through its respective manifold nipple. The thermocouple on the nipple of the involved coil will then read the average value of the two adjacent coil thermocouples, thus indicating a normal reading.
Mixing of all coil discharge water occurs in the discharge water manifold, resulting in a large temperature gradient along the length of the manifold nipple. This temperature gradient is an important consideration when connecting the thermocouple to the nipple. Placement of each nipple thermocouple must be at the same place relative to the length of the nipple which should result in very little difference between all thermocouple readings. However, this installation accuracy is never obtained in actual practice and in conjunction with other variables, the normal build in temperature difference between thermocouples can be as much as 10.degree. to 12.degree. C.
The practice of using redundant thermocouples is becoming more common and closer installation tolerances are that much more important. The redundant thermocouples must be placed at the same distances along the length of the nipple in order to read as close as possible to the same temperature for the one coil.
Also, the stator water discharge manifold is exposed to the ambient temperature of the generator's hydrogen gas. Unbalance conditions in the generator coolers can cause a temperature gradient to occur on the discharge water manifold which could cause significant temperature differences in the stator coil temperature readings.
In summary, the major problems associated with the current prior art method of monitoring stator coil discharge water temperature are as follows. First, the location of the thermocouple along the length of the nipple is critical with respect to all nipples because of the temperature gradient along the nipple. Second, the nipple mounted thermocouple will not detect a change in temperature resulting from a complete blockage of water flow to a coil. Third, since the discharge water manifold is in the ambient cool gas, a hydrogen cooler unbalance condition could cause a stator coil temperature change or abnormal temperature indication. Fourth, the existing prior art method does not monitor the true temperature of the coil's discharge water because of the thermocouple's close proximity to the discharge water manifold. Fifth, placement of redundant thermocouples is very critical in order for each to read the same temperature. And, sixth, factory installation of thermocouples following generator construction makes it difficult to obtain a precise mounting of the thermocouple on the manifold nipple.
Consequently, a need still remains for an alternative way of monitoring the temperatures of water-cooled stator windings of an electric generator.